Metal-oxide-semiconductor field-effect phototransistors based on single-crystalline semiconductor thin films

ABSTRACT

MOSFET phototransistors, methods of operating the MOSFET phototransistors and methods of making the MOSFET phototransistors are provided. The phototransistors have a buried electrode configuration, which makes it possible to irradiate the entire surface areas of the radiation-receiving surfaces of the phototransistors.

BACKGROUND

Photodetectors are indispensable components for most optoelectronicapplications due to their ability to convert light signals intoelectrical signals. Compared with the renowned III-V compound-basedphotodetectors, silicon (Si)-based photodetectors can be easilyintegrated with the conventional Si-basedcomplementary-metal-oxide-semiconductor (CMOS) technology. Althoughconventional p-n and p-i-n photodiodes are popular device structures,they suffer from low responsivity and quantum efficiency. Therefore, itis necessary to boost their electrical signals to an acceptable rangevia amplifiers.

Metal-oxide-semiconductor field-effect-transistors (MOSFETs) exhibitexceptional photo sensing capability. MOSFET type photodetectors, alsoknown as phototransistors, have the advantages of not only high photosensitivity and responsivity, but also integrability into conventionalCMOS chips. However, some shortcomings still exist due to structurallimitations, such as limited light-sensing area and light blocking bygate electrodes. More importantly, these devices are built on rigidsubstrates, making it difficult to manipulate the physical shapes of thephotodetectors.

SUMMARY

MOSFET phototransistors, methods of operating the MOSFETphototransistors and methods of making the MOSFET phototransistors areprovided.

One embodiment of a MOSFET phototransistor comprises: a substrate; asingle-crystalline semiconductor film comprising: a radiation-receivingsurface; an opposing, substrate-facing surface; a source region; a drainregion; and a channel region; a gate stack comprising a gate dielectricand a gate electrode, wherein the gate stack is disposed on thesubstrate-facing surface of the single-crystalline semiconductor film,between the channel region of the single-crystalline semiconductor filmand the substrate; a source electrode disposed on the substrate-facingsurface of the single-crystalline semiconductor film, between the sourceregion of the single-crystalline semiconductor film and the substrate; adrain electrode disposed on the substrate-facing surface of thesingle-crystalline semiconductor film, between the drain region of thesingle-crystalline semiconductor film and the substrate; and anantireflective coating on the radiation-receiving surface of thesingle-crystalline semiconductor film.

One embodiment of a method of operating the MOSFET phototransistorcomprises applying a gate voltage to the gate electrode; and irradiatingthe radiation-receiving surface of the single-crystalline semiconductorfilm with incident radiation, whereby charge carriers are created in thesingle-crystalline semiconductor layer. This results in the modulationof the gate voltage and the drain current in the phototransistor, whichcan be measured using a current measuring device.

One embodiment of a method for making a MOSFET phototransistorcomprises: forming a source region, a drain region, and a channel regionin a single-crystalline semiconductor film that is attached to asacrificial substrate, the single-crystalline semiconductor substratecomprising a first surface and a second surface facing opposite thefirst surface; releasing the single-crystalline semiconductor film fromthe sacrificial substrate; forming a source electrode over the sourceregion on the first surface of the single-crystalline semiconductorfilm; forming a drain electrode over the drain region on the firstsurface of the single-crystalline semiconductor film; and forming a gatestack comprising a gate dielectric and a gate electrode over the channelregion on the first surface of the single-crystalline semiconductorfilm; attaching the released single-crystalline semiconductor film ontoa support substrate, with the source electrode, gate stack, and drainelectrode facing the support substrate, such that second surface of thesingle-crystalline semiconductor film is facing away from the supportsubstrate; and applying an antireflective coating over the secondsurface of the single-crystalline semiconductor film.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIG. 1A. Schematic cross-sectional diagram of ametal-oxide-semiconductor field-effect transistor (MOSFET)phototransistor. FIG. 1B. Enlarged view of the central portion of thephototransistor of FIG. 1A.

FIG. 2. Schematic diagram of the process flow for the fabrication of thephototransistor of FIG. 1A.

FIGS. 3A-F. Schematic illustration of the process flow for asilicon-based phototransistor in accordance with the Example. FIG. 3A.Formation of n⁺ wells by ion implantation to define the source/drainregions to achieve an ohmic contact. FIG. 3B. Release of the siliconnanomembrane (Si NM) by selective etching of a buried oxide layer. FIG.3C. Metallization by e-beam evaporation to deposit source/drainelectrodes, and a gate stack comprising a gate electrode and gatedielectric. FIG. 3D. Transfer printing of a fully fabricated device ontoan adhesive layer-coated PET substrate, FIG. 3E. Spin-coating of aprotective, antireflection layer on top of the radiation-receivingsurface. FIG. 3F. Microscopic image of the finished flexiblephototransistor. Insets in FIGS. 3C and 3E depict the cross-sectionalview of device.

FIG. 4A. Drain current-gate voltage characteristics (I_(DS)-V_(GS)) atV_(DS)-0 V for the phototransistor of the Example. Inset shows themagnified plot of photo currents. FIG. 4B. Drain current-drain voltagecharacteristics (I_(DS)-V_(GS)) under dark and illumination with variouslight sources (red, green, and blue) illuminating the flexiblephototransistors. The I_(DS)-V_(GS) curve under V_(GS)=1 V shows aphoto-to-dark current ratio as high as ˜1×10⁵. FIG. 4C. Three layerstructures used to simulate the light absorption of the Si NM of thephototransistor. FIG. 4D. The corresponding simulated absorption of theSi NM: (i) without any layers; (ii) with the metal reflector underneaththe Si NM; and (iii) with the metal reflector underneath the Si NM andSU-8 ARC layer on top of Si NM, respectively. Dashed line denotes blue(473 nm), green (532 nm), and red (632 nm) wavelengths.

FIG. 5A-D. Drain current-drain voltage characteristics (I_(DS)-V_(DS))with different V_(GS) biases ranging from 0 to 1 V with a 0.1 V stepunder the dark and the green light illumination of flexiblephototransistors in the Example, which have different W/L ratios: FIG.5A. W=25 μm, L=2 μm; FIG. 5B. W=50 μm, L=2 μm; FIG. 5C. W=50 μm, L=10μm; FIG. 5D. W=100 μm, L=10 μm.

FIG. 6A. The trend of responsivity with respect to channel lengths from2 to 10 μm for the phototransistor of the Example. The channel width andbias point was fixed at 50 μm and at V_(GS)=1 V and V_(DS)=3 V. FIG. 6B.Responsivity under bending conditions measured at a fixed voltage biaspoint of V_(GS)=1 V and V_(DS)=3 V. FIG. 6C. An optical image of theflexible Si NM phototransistor on a bent substrate. FIG. 6D. Modulatedphotocurrent under pulsed 5 mW green laser measured at a fixed voltagebias of V_(GS)=1 V and V_(DS)=3 V.

FIG. 7. Image of a polymer lens pattern that can used for a patternedanti-reflection coating.

DETAILED DESCRIPTION

MOSFET phototransistors, methods of operating the MOSFETphototransistors and methods of making the MOSFET phototransistors areprovided.

The phototransistors have a buried electrode configuration, which makesit possible to irradiate the entire surface areas of theradiation-receiving surfaces of the phototransistors, thereby increasingtheir radiation absorption. The buried electrode configuration can becombined with an antireflective coating on the radiation-receivingsurface and radiation-reflective backside electrode to further enhancethe performance of the devices-providing them with high lightsensitivities and stable performance. Additionally, because thephototransistors can be fabricated from thin transferrablesingle-crystalline semiconductor films (also referred to assemiconductor nanomembranes) on polymeric substrates, they can bemechanically flexible. As a result, the phototransistors haveapplications in high performance flexible optical sensors,photodetectors and CMOS imagers.

An embodiment of a MOSFET phototransistor 100 is shown in FIG. 1A. Thephototransistor includes a substrate 102, which supports the activecomponents of the phototransistor. In the embodiment shown here,substrate 102 comprises a base substrate 104 with an adhesive coating106 on its upper surface. The active region of the phototransistor iscomprised of a single-crystalline semiconductor film 110 comprising: aradiation-receiving surface 112; an opposing, substrate-facing surface114; a source region 116, a drain region 118 and a channel region 120. Agate stack comprising a gate dielectric 122 and a gate electrode 124 isdisposed on substrate-facing surface 114 and positioned betweensemiconductor film 110 and substrate 102. Similarly, a source electrode126 and a drain electrode 128, disposed on source region 116 and drainregion 119, respectively, are positioned between semiconductor film 110and substrate 102. In this configuration, adhesive coating 106, whichmay be an epoxy polymer, helps to attach the active components of thephototransistor to base substrate 104. An antireflective coating (ARC)130 is provided over radiation-receiving surface 112.

In some embodiments, the ARC is patterned in order to increase theincident radiation collection efficiency of the phototransistor,relative to the incident radiation collection efficiency of an ARChaving planar surfaces. For example, a radiation focusing pattern can bedefined into one or both of the upper (radiation-receiving) and lower(semiconductor film-facing) surfaces of the ARC in order to redirect andfocus more radiation into the channel region of the phototransistor. Forexample, the surfaces may be faceted and/or comprise a microscale ornanoscale patterned surface topography, that is—a surface topographyhaving features with one or more dimensions of about 100 μm or less andabout 100 nm or less, respectively. Examples of patterns that can beused to increase the radiation collection efficiency include arrays ofanti-rings and arrays of nanopillars, including pointed cone-shapedpillars and rounded cone-shaped pillars, as describe in Po-Yuan Chen etal., Opt. Express 22, A1128-A1136 (2014) and Seungmuk Ji et al.,Nanoscale, 4, 4603-4610 (2012). Arrays of dome-shaped lenses andnanospheres and Fresnel lenses can also be used. In some embodiments,the pattern comprises a plurality of plano-convex microlenses extendingupward from the upper surface of the ARC. Such microlenses can beformed, for example via polymer reflow of a plurality of polymericmicrocylinders patterned on or defined in the upper surface of the ARC.By way of illustration, FIG. 7 is an image of a polymer lens array thatcan be fabricating using a polymer reflow technique.

FIG. 1B provides an enlarged view of the central portion ofphototransistor 100 and illustrates the role of the ARC and gateelectrode in enhancing radiation absorption by semiconductor film 110.As shown in the figure, when incident radiation comprising a pluralityof wavelengths 132 impinges on radiation-receiving surface 112, aportion of the incident radiation 134 may be reflected away fromsemiconductor film 110. However, by including ARC 130 the loss due toreflection can be decreased, as ARC 130 operates to reflect radiation134 back to radiation-receiving surface 112 of semiconductor film 110.

The ARC material should be selected such that it is opticallytransparent to the incident radiation to be absorbed by thesingle-crystalline semiconductor film and has an index of refractionsuitable for achieving the desired reflectance. Epoxy polymers areexamples of materials that can be used as ARCs. Polymeric ARCs areadvantageous because they are mechanically flexible. However, othermaterials, including inorganic materials, such as indium tin oxide (ITO)may also be employed. The ARC can be quite thin, having a thickness of 1μm or less, including thicknesses of 0.5 μm or less. However, thickerARCs can be used.

As shown in FIGS. 1A and 1B, one or more of the source, drain, and gateelectrodes can be positioned below the single-crystalline semiconductorlayer, such that incident radiation 132 that traverses ARC 130 andsingle-crystalline semiconductor layer 110 to impinge on an electrode(124, 126 or 128) is reflected back into single-crystallinesemiconductor layer 110, where it may be absorbed to producephotogenerated carriers. In order to maximize the reflectance from theelectrodes, the electrode material—typically a metal—desirably has ahigh reflectivity for the incident radiation. In some embodiments of thephototransistors, the electrodes comprise aluminum or silver.

The source and drain regions are formed by selectively doping thesingle-crystalline semiconductor film, which is itself a dopedsemiconductor. The MOSFET phototransistors can be n-channel MOSFETs, inwhich the source and drain regions are formed by n-type doping a p-typesingle-crystalline semiconductor film. Alternatively, the MOSFETphototransistors can be p-channel MOSFETs, in which the source and drainregions are formed by p-type doping an n-type single-crystallinesemiconductor film.

The single-crystalline semiconductor film is desirably sufficiently thinto be mechanically flexible, such that, in conjunction with the otherlayers and components of the device, it provides a mechanically flexiblephototransistor. By way of illustration, in some embodiments of thephototransistors, the single-crystalline semiconductor film has athickness no greater than 500 nm. This includes embodiments in which thesingle-crystalline semiconductor film has a thickness no greater than400 nm and further includes embodiments in which the single-crystallinesemiconductor film has a thickness no greater than 300 nm. For example,the thickness of the single-crystalline semiconductor film can be in therange from about 100 nm to about 300 nm.

Suitable semiconductors for the semiconductor film include inorganicGroup IV semiconductors, Group III-V semiconductors and Group II-VIsemiconductors. The semiconductors may be single element semiconductors,such as silicon and germanium, or may be alloyed or compoundsemiconductors, such as SiGe, GaAs, InGaAs and InGaP. In addition, thesemiconductor thin film may be strained or unstrained and may comprisemore than one sublayer, provided at least one sublayer is asingle-crystalline semiconductor film. For example, strained multilayersemiconductor films of the type described in U.S. Pat. No. 7,973,336,the disclosure of which is incorporated herein by reference, can be usedas the single-crystalline semiconductor film. (Briefly, such strainedmultilayer semiconductor films comprise a partially compressivestrain-relaxed single-crystalline SiGe film sandwiched between a firsttensilely strained single-crystalline silicon film and a secondtensilely strained single-crystalline silicon film.) The selection ofsemiconductor material will depend, at least in part, on thephototransistor's desired wavelength range of operation. For example,silicon is photoactive in the UV, visible and near-infrared regions ofthe electromagnetic spectrum (e.g., from wavelengths around 200 nm toaround 1100 nm).

The substrate may be a rigid substrate, such as a semiconductor wafer.However, flexible substrates are preferred for flexible deviceapplications. Examples of flexible substrates include polymericsubstrates and thin inorganic substrates. Polyethylene terephthalate isan example of a polymer from which the substrate can be comprised. Othersuitable polymers include: polyethylene naphthalate (PEN);polyestersulfone (PES); and polyimide (PI).

In some embodiments, all of the layers and components in thephototransistors are sufficiently thin and/or comprised of sufficientlyflexible materials to render the phototransistors mechanically flexible.By way of illustration, the substrate, including any adhesive coating,may have a thickness of no greater than 110 μm and the entirephototransistor from the radiation-receiving surface of the ARC to thebottom surface of the substrate may have a thickness of less than 120μm. For the purposes of this disclosure, a phototransistor ismechanically flexible if it can be bent to a radius of curvature of 15mm, without undergoing a decrease in its responsivity of 10% or greater,as measured according to the procedures described in the Example. Asillustrated in the Example, some embodiments of the phototransistors areable to bend to a radius of curvature of 15 mm with less than a 5%decrease in responsivity.

Although not shown in FIG. 1A, connections between the electrodes andexternal circuitry and/or devices can be made from the backside of thephototransistor, through the substrate.

The MOSFET phototransistor operates as follows. When radiation falls onthe surface of the semiconductor film, photons having energies greaterthan the band-gap energy of the semiconductor material, are absorbed.This absorption results in the photogeneration of charge carries(electron/hole pairs), which can travel through the semiconductor filmunder an intrinsic or externally-applied electric field. The continuousseparation of the photogenerated electron-hole pairs produces aphotogenerated drain current, the magnitude of which is proportional tothe intensity of the incident radiation.

In the phototransistor, the drain current increases under opticalillumination because the incident radiation creates excess electron holepairs in the depletion region. Because of this, a photovoltage develops,which modifies the effective gate bias, enhancing the transistorconductivity and increasing the drain current.

The phototransistors can be operated in two modes. In the first mode,low voltage bias conditions are used to achieve a very high photocurrentto dark current ratio. The parameters of the low voltage conditions willdepend on the nature of the particular phototransistor being tested.However, in some embodiments of the phototransistors very highphotocurrent to dark current ratios can be achieved using a gate voltage(V_(GS)) of about 0.5 V or lower and a drain voltage (V_(DS)) of about50 mV or lower. Using the first mode of operation, photocurrent to darkcurrent ratios of at least 10³, at least 10⁴, and at least 10⁵ can beachieved when the photocurrent is measured using visible light as theincident radiation, as illustrated in the Example.

In the second mode, higher bias conditions are used to achieve a highincident radiation responsivity. The parameters of the high voltageconditions will depend on the nature of the particular phototransistorbeing tested. However, in some embodiments of the phototransistors highresponsivities can be achieved using a gate voltage (V_(GS)) of about 1V or higher and a drain voltage (V_(DS)) of about 3 V or higher. Usingthe second mode of operation, responsivities of at least 10 A/W, atleast 40 A/W and at least 50 A/W can be achieved, as illustrated in theExample.

The phototransistors can be fabricated via the release and transfer ofsingle-crystalline semiconductor films grown on sacrificial substrates.For example, they can be fabricated from the device layer of asemiconductor (e.g., Si)-on-insulator (SOI) substrate, wherein theburied oxide of the SOI is used as a sacrificial substrate. Thefabrication process is illustrated schematically in FIG. 2. The processbegins (step (a)) with an SOI 200 comprising a handle substrate 202,such as a silicon wafer, a sacrificial layer 204, such as a buriedsilicon oxide layer, and a single-crystalline semiconductor layer 110,into which a source region 116, a drain region 118 and a channel region120 have been defined via selective doping. Next, sacrificial layer 204is removed (for example, chemically etched away) to releasesemiconductor film 110 (step (b)). A source electrode 126 and a drainelectrode 128 are deposited on source region 116 and drain region 118,respectively. A gate stack comprising a gate oxide 122 and a gateelectrode 124 is formed on channel region 120 (step (c)). The releaseddevice is then transferred—electrode and gate stack side-down—onto asupport substrate 104, which includes an adhesive coating 106 on itsupper surface (step (d)). As a result, the radiation-receiving surface112 of single-crystalline semiconductor film 110 can be fully exposed toincident radiation, without any radiation blocking from the source anddrain electrodes, the gate stack, or any other device components. Oncesemiconductor film 110 is released and transferred away from handlesubstrate 202, another layer of sacrificial material can be grown on itsnewly exposed surface. Then the process can be repeated to fabricateanother MOSFET. In this manner, the wafer substrate can be continuallyrecycled in the fabrication of a plurality of MOSFETs.

Example

In this Example, flexible phototransistors with a back gateconfiguration based on transferrable single-crystalline Si nanomembrane(Si NM) are demonstrated. Having the Si NM as the top layer enables fullexposure of the active region to incident light and, thus, allows foreffective light sensing. The flexible phototransistors operate in twomodes: (1) the high light detection mode, which exhibits a photo-to-darkcurrent ratio of 10⁵ at voltage bias of V_(GS)<0.5 V and V_(DS)=50 mV;and (2) the high responsivity mode, which shows a maximum responsivityof 52 A/W under blue illumination at voltage bias of V_(GS)=1 V andV_(DS)=3 V. Due to the good mechanical flexibility of Si NMs, with theassistance of a polymer layer to enhance light absorption, the deviceexhibited stable responsivity with less than 5% variation under bendingat small radii of curvatures (up to 15 mm). The flexiblephototransistors with the capabilities of high sensitivity lightdetection and stable performance under the bending conditions can beused in high performance flexible optical sensor applications, with easyintegration for multi-functional applications.

This Example illustrates the fabrication and performance characteristicsof thin-film transistor type flexible phototransistors usingsingle-crystalline Si NMs. The phototransistors were designed tomaximize light sensing by the flip transfer of the Si NM layer, the gatestack, and source/drain electrodes (resulting in a geometry that isreferred to herein as a silicon-on-top structure). In this structure,unlike other MOSFET-type photodetectors, light-absorption in a Si NMlayer can be much more efficient because light is not blocked by anymetal layers or other materials. Also, the active region, where light isdetected, is not limited by channel dimension. Moreover, electrodesplaced underneath the Si NM also acted as light reflectors to furtherimprove light absorption. These design merits led to very highresponsivity, despite the use of thin Si NMs (i.e., only 270 nm).Overall, the flexible Si NM phototransistors exhibited not only asufficient light/dark current ratio when working at a very low voltagebias, but also very high responsivity when working at a normal operationvoltage bias.

Results and Discussion

The schematic illustration of the device fabrication is shown in FIG. 3.The fabrication details can be found in the experimental section, but inshort, the process began with n+ doping the top Si device layer of a SOIwafer (SOITEC) to define source and drain regions (FIG. 3A), followed byrelease of the top Si device layer (Si NM) (FIG. 3B). Gate andsource/drain electrodes were deposited by e-beam evaporation (FIG. 3C).After the metallization step, all of the device layers (metal electrodesand Si NM) were flip transferred on to an adhesive layer (SU-8 2002,Microchem) coated on a polyethylene terephthalate (PET) substrate andcured (FIG. 3D). The active layer was defined (FIG. 3E) and covered byan anti-reflection coating (ARC) layer (SU-8 2002). The ARC layer has arefractive index (n) between 1.57 and 1.65 for visible light with theaverage optical transmittance in the visible range of nearly 97%. (See,P. Krogstrup, H. I. Jorgensen, M. Heiss, O. Demichel, J. V. Holm, M.Aagesen, J. Nygard A. F. i Morral, Nature Photon. 2013, 7, 306-310 andMicroChem, SU-8 2000.5-2015 Data Sheet,http://www.microchem.com/Prod-SU82000.htm.) Thus, it reduces lightreflection and enhances light transmission into the Si NM and,consequently, improves light absorption by the Si NM. Detaileddescriptions on the transfer printing process and ion implantation canbe found elsewhere. (See, W. Zhou, D. Zhao, Y.-C. Shuai, H. Yang, S.Chuwongin, A. Chadha, J.-H. Seo, K. X. Wang, V. Liu, Z. Ma, S. Fan,Prog. Quant. Electron. 2014, 38, (1) 1-74.) Shown in FIG. 3F is amicrograph of the fabricated phototransistor. It can be clearly seenthat the Si NM, where light is sensed, lies on top of the gate stack andthe source/drain electrodes. Therefore, the incident light is notblocked by any of these layers.

The light sensing characteristics were measured under dark andilluminated conditions by using representative Si NM phototransistors,which had a channel width and channel length of 50 μm and 2 μm,respectively. As shown in FIG. 4A, the phototransistor can be operatedin two different modes: the first mode is for high photo-to-dark currentratio, achieved by operating under low voltage biases; and the secondmode is for high responsivity, achieved under high voltage biases. Inthe first mode, the phototransistors showed extremely low dark current(<10 pA) and high photocurrent (i.e., drain current) (>0.4 μA) undervery low voltage bias (V_(GS)<0.5 V, and V_(DS)=50 mV) which yielded aphoto-to-dark current ratio of up to 10⁵. As gate bias increased, thetransistor current substantially increased, due to the transistoreffect, and the photo-to-dark current ratio thus decreased to 11. Thisindicates that the devices are suitable for operation in the first modeunder very low bias. The very high photo-to-dark current ratio isattributed to the silicon-on-top structure, better light absorption withthe SU-8 ARC layer, and the use of the metal electrodes as backsidereflectors.

To prove the effectiveness of light absorption enhancement of the ARCand the metal electrode reflectors, optical simulations were performed.FIG. 4D shows the simulated light absorptions of the Si NM in thevisible wavelength range (400 nm˜700 nm) before adding any lightabsorption-assisting layers (Case 1), after applying the metal electrodeas a reflector (Case 2), and after applying both the SU-8 ARC layer andthe metal electrode reflector (Case 3). The three cases are illustratedschematically in FIG. 4C. It should be noted that the simulations wereperformed based on the actual layer thicknesses and optical parameters.The averaged light absorption of the Si NM in the visible wavelengthregion nearly doubled, from 20.2% to 38.3%, with the metal reflector onthe backside of the Si NM. In addition, the SU-8 ARC layer on top of theSi NM further enhanced its average light absorption from 38.3% to 48%,which is equivalent to using a 480 nm thick Si NM. Particularly, lightabsorption of blue, green, and red wavelengths by the Si NM increasedfrom 44%, 20%, and 5% to 99%, 99%, and 40%, respectively, by adding themetal reflector and the ARC layer.

In FIG. 4B, I_(DS)-V_(DS) characteristics under red, green, and bluelaser illumination are shown. When the phototransistors were biased athigher voltages, they operated in the second mode, where highresponsivity was achieved. Responsivity “R” in phototransistors can becalculated from the equation: (1) R=I_(ph)/P_(inc) I_(ph)/(E=A), whereI_(ph) is the photocurrent, P_(inc) is the light power incident onto theactive region of the device, E is the irradiance of the incident light(power intensity), and A is the area of the active region. (See, Z.Huang, J. E. Carey, M. Liu, X. Guo, E. Mazur, J. C. Campbell, Appl.Phys. Lett. 2006, 89, 033506.) According to Eq. (1), when the incidentlight was reflected back to the active region by the bottom metalreflector and the top ARC layer, the light absorption by the activeregion nearly doubled, which is equivalent to half of the P value.Therefore, the enhanced light absorption in the active region is a majorfactor in the improved responsivity of the devices. In this calculation,˜4 mW/mm² of E and 2000 μm² of A were used. It should be noted that thearea of the active region refers to the top surface area of the entireSi NM, including the channel region and contact region, due to the SiNM-on-top structure. The responsivity at a voltage bias of V_(GS)=1 Vand V_(DS)=3 V under red, green, and blue light illumination werecalculated to be 51, 41, and 18 A/W, respectively. It should be notedthat the responsivity under low gate bias, which is the first mode, wasonly 0.04 A/W. Considering the responsivity value of ˜1 A/W from anamorphous Si phototransistor deposited on a glass substrate having asimilar layer thickness to our Si NM phototransistor (see, Y.Vygranenko, A. Nathan, M. Vieira, and A. Sazonov, Appl. Phys. Lett.2010, 96, 173507), these responsivity values are relatively highconsidering the thin Si NM used in the device. The increasing spectralresponsivities at decreasing wavelengths of light may be attributed tothe increase in absorption coefficients of Si in the visible lightrange. To investigate the effect on photocurrent with various dimensionsin the Si NM active regions, phototransistors with different channelareas (W/L=25/2 μm, 50/2 μm, 50/10 μm, 100/10 μm) were measured underdark and green light illumination conditions (532 nm), as shown in FIG.5A, FIG. 5B, FIG. 5C and FIG. 5D, respectively. As expected from theequation: (2) I_(D)=0.5

_(ox) (

/L) (

_(GS) V—_(t))², where W and L are the channel width and length, thedevices with larger W/L ratios showed higher photocurrents. (See, S. M.Sze and K. K. Ng, Physics of Semiconductor Devices, 3rd ed. (Wiley, NewYork, 2006).) It was also observed that the devices with larger L valuesshowed better light sensitivity, if the W/L ratio was fixed. As shown inthe devices of FIG. 5A and FIG. 5D, which have similar W/L ratios, thedevice with L=10 μm showed 2.35 times the photo-to-dark currentincrement at V_(GS)=1 V and V_(DS)=3 V, while the device with L=2 μmonly showed 1.3 times the increment.

The responsivities under red, green, and blue light illumination as afunction of channel length are shown in FIG. 6A. Responsivitiesdecreased with an increase in channel length from 2 μm to 10 μm. Thetrend agrees well with the relationship between the channel current andthe responsivity given by Eq. (3): R=0.5

_(ox) (

_(GS)

_(th))²/(L²

), by incorporating Eq. (1) and Eq. (2). Therefore, the device with asmaller channel length was more suitable for the second operation mode,which is for high responsivity. FIG. 6B shows the responsivity measuredusing red (632 nm), green (532 nm), and blue (473 nm) laser sourcesunder the bending condition of external uniaxial strains of 1.08% and0.2% for tensile and compressive strain, respectively (measured frombending curvature). An optical image of the flexible Si NMphototransistor on a bent substrate is shown in FIG. 6C. Theresponsivities were measured from the device whose W/L was 50/2 μm undergreen light. It is speculated that the increase in the responsivity isattributed to the mobility enhancement in the crystal structure of theSi NM. The bendable characteristics of the phototransistors make themsuitable for a number of important applications, such as wide view angleimaging.

The drain current as a function of time is shown in FIG. 6D. A 1millisecond (msec) pulse mode green laser was used as a light source andthe waveform was recorded by a digital oscilloscope (TektronicsTDS1000C). We used the same device and light sources for the bendingexperiment. The device showed 0.05 msec and 0.11 msec rise and falltimes. No overshooting and oscillating were observed.

Experimental Section Device Fabrication

The fabrication began with an SOI wafer (SOITEC) which had a 270 nmlightly p-type doped (boron, ˜1×10¹⁵ cm⁻³) top Si device layer and a 200nm buried oxide layer. Two n⁺ well regions (˜8×10¹⁹ cm⁻³) were formed byphosphorous ion implantation, followed by a diffusion process at 850° C.for 40 minutes (FIG. 3A). After forming holes through the top Si devicelayer, now called the Si NM, it was released from the buried oxide layerby selective wet-etching in concentrated HF (49%). The released top SiNM gently fell down onto the handling Si substrate (FIG. 3B). Afterrinsing and drying the sample, the Si NM was weakly bonded to thehandling Si substrate via Van der Waals forces. Gate stack (SiO₂/Ti/Au100 nm/20 nm/80 nm) and source/drain electrodes (Ti/Au 20 nm/180 nm)were patterned using e-beam lithography and deposited by e-beamevaporation (FIG. 3C). After the completion of the metallization step,all of the device layers (metal electrodes and Si NM) were fliptransferred to an 0.6 μm thick adhesive layer (SU-8 2002, Microchem)coated on a polyethylene terephthalate (PET) substrate, and cured toglue the devices onto the PET substrate (FIG. 3D). After the transferstep, the Si NM layer was positioned face-up and the electrodes wereburied underneath the Si NM. The active layer was defined by thephotoresist patterning and dry etching steps (FIG. 3E). Finally, theentire device was covered by an anti-reflection coating (ARC) layer(SU-8 2002).

Improved Transfer Printing Method:

With the conventional transfer printing technique, the semiconductornanomembrane can be transferred on either a flat or a curvy surface witha yield of nearly 100%. (See, J. A. Rogers, T. Someya, and Y. Huang,Science 2010, 327, 5973 and J. A. Rogers, M. G. Lagally, and R. G.Nuzzo, Nature 2011, 477, 45.) However, the position of the transferredsemiconductor nanomembrane can be slightly changed during thefabrication process due to the different thermal expansions between theplastic substrate and the adhesion layer. (See, L. Sun, G. Qin, J.-H.Seo, G. K. Celler, W. Zhou, Z. Ma, Small 2010, 6, (22), 2553-2557.) Theimproved transfer printing technique we used in this work, i.e.releasing a Si nanomembrane and then transfer printing the finisheddevices, enables one to avoid such issues effectively. The criticalparameter in order to use this method is to have a thin buried oxidelayer, so as to increase the Van der Waals force to hold the released Sinanomembrane during the rest of fabrication process. It was discoveredthat the new method can be used for a BOX layer with a thickness thinnerthan 200 nm.

Device Characterization:

I-V characteristics of the fabricated devices under dark andillumination were measured using a semiconductor parameter analyzer(HP4155B). The photosensitivity of the devices was also characterizedusing three different laser sources (473 (blue), 532 (green), and 632 nm(red) wavelengths) with the same power intensity of 4 mW/mm².

Simulation of Absorption in Si NM:

The simulation was carried out by using a three-dimensionalfinite-difference time-domain (3D FDTD) technique in the visible lightregion (400 nm to 700 nm) where the refractive indices of Si NM and SU-8used in the simulation were 3.48 and 1.57-1.65 for visible lightwavelength, respectively. For simplicity and numerical expediency, welimited the study to surface normal incidence. We also assumed that thesize and thickness of designed pattern were same as the size of theactive region.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A metal-oxide-semiconductor field-effectphototransistor comprising: a substrate; a single-crystallinesemiconductor film comprising: a radiation-receiving surface; anopposing, substrate-facing surface; a source region; a drain region; anda channel region; a gate stack comprising a gate dielectric and a gateelectrode, wherein the gate stack is disposed on the substrate-facingsurface of the single-crystalline semiconductor film, between thechannel region of the single-crystalline semiconductor film and thesubstrate; a source electrode disposed on the substrate-facing surfaceof the single-crystalline semiconductor film, between the source regionof the single-crystalline semiconductor film and the substrate; a drainelectrode disposed on the substrate-facing surface of thesingle-crystalline semiconductor film, between the drain region of thesingle-crystalline semiconductor film and the substrate; and anantireflective coating on the radiation-receiving surface of thesingle-crystalline semiconductor film.
 2. The phototransistor of claim1, wherein at least one of the source electrode, gate electrode anddrain electrode is configured to reflect radiation that traverses thesingle-crystalline semiconductor film back into the single-crystallinesemiconductor film.
 3. The phototransistor of claim 2, wherein each ofthe source electrode, gate electrode and drain electrode are configuredto reflect radiation that traverses the single-crystalline semiconductorfilm back into the single-crystalline semiconductor film.
 4. Thephototransistor of claim 1, wherein the substrate is a polymericsubstrate and the phototransistor is mechanically flexible.
 5. Thephototransistor of claim 1, wherein a surface of the substrate incontact with the source electrode, gate electrode and drain electrodecomprises an adhesive coating.
 6. The phototransistor of claim 1,wherein the antireflective coating and the adhesive coating bothcomprise an epoxy polymer.
 7. The phototransistor of claim 1, whereinthe antireflective coating defines a pattern configured to increase theincident radiation collection efficiency of the phototransistor,relative to the incident radiation collection efficiency provided by anunpatterned antireflective coating.
 8. The phototransistor of claim 7,wherein the substrate is a mechanically flexible, polymeric substrate.9. The phototransistor of claim 8, wherein a surface of the substrate incontact with the source electrode, gate electrode and drain electrodecomprises an adhesive coating.
 10. The phototransistor of claim 1,wherein the single-crystalline semiconductor film is asingle-crystalline Group IV semiconductor film.
 11. The phototransistorof claim 10, wherein the single-crystalline semiconductor film is asingle-crystalline silicon film.
 12. The phototransistor of claim 1,wherein the single-crystalline semiconductor film has a thickness of nogreater than 400 nm.
 13. The phototransistor of claim 1, wherein thesource electrode, gate electrode, and drain electrode comprise aluminumor silver.
 14. A method of controlling the drain current in ametal-oxide-semiconductor field-effect phototransistor comprising: asubstrate; a single-crystalline semiconductor film comprising: aradiation-receiving surface; an opposing, substrate-facing surface; asource region; a drain region; and a channel region; a gate stackcomprising a gate dielectric and a gate electrode, wherein the gatestack is disposed on the substrate-facing surface of thesingle-crystalline semiconductor film, between the channel region of thesingle-crystalline semiconductor film and the substrate; a sourceelectrode disposed on the substrate-facing surface of thesingle-crystalline semiconductor film, between the source region of thesingle-crystalline semiconductor film and the substrate; a drainelectrode disposed on the substrate-facing surface of thesingle-crystalline semiconductor film, between the drain region of thesingle-crystalline semiconductor film and the substrate; and anantireflective coating on the radiation-receiving surface of thesingle-crystalline semiconductor film; the method comprising: applying agate voltage to the gate electrode; and irradiating theradiation-receiving surface of the single-crystalline semiconductor filmwith incident radiation, whereby charge carriers are created in thesingle-crystalline semiconductor layer, which modulates the gate voltageand the drain current in the phototransistor.
 15. The method of claim14, wherein the photocurrent to dark current ratio of thephototransistor is at least 10⁴.
 16. The method of claim 14, wherein thephototransistor has a responsivity of at least 40 A/W.
 17. A method offabricating a metal-oxide-semiconductor field-effect phototransistor,the method comprising: forming a source region, a drain region, and achannel region in a single-crystalline semiconductor film that isattached to a sacrificial substrate, the single-crystallinesemiconductor substrate comprising a first surface and a second surfacefacing opposite the first surface; releasing the single-crystallinesemiconductor film from the sacrificial substrate; forming a sourceelectrode over the source region on the first surface of thesingle-crystalline semiconductor film; forming a drain electrode overthe drain region on the first surface of the single-crystallinesemiconductor film; and forming a gate stack comprising a gatedielectric and a gate electrode over the channel region on the firstsurface of the single-crystalline semiconductor film; attaching thereleased single-crystalline semiconductor film onto a support substrate,with the source electrode, gate stack, and drain electrode facing thesupport substrate, such that second surface of the single-crystallinesemiconductor film is facing away from the support substrate; andapplying an antireflective coating over the second surface of thesingle-crystalline semiconductor film.
 18. The method of claim 17,wherein the support substrate is a polymeric substrate and thephototransistor is mechanically flexible.
 19. The method of claim 17,wherein the antireflective coating defines a pattern configured toincrease the incident radiation collection efficiency of thephototransistor, relative to the incident radiation collectionefficiency provided by an unpatterned antireflective coating.